Radio signal transmission systems typically use single or small antenna groups to properly transmit/receive radio signals. Multi-antenna technology can improve space resource utilization, increase radio channel bandwidth efficiency, and improve system capacity. Compared with traditional antenna techniques, multiple-input and multiple-output, or MIMO, antennas systems use multiple antennas at both the transmitter and receiver to improve communication performance. MIMO technology has an advantage of improving channel capacity. Two antenna MIMO technology has been widely used, for example a macro node (eNodeB) using two antennas to transmit downlink (DL) signals and receive uplink signals (UL).
In long term evolution (LTE) communication systems, two different frame structures exist. Of the two, frame type-2 structures are applicable to time division duplex (TDD), and at least for LTE high speed data transmission, may require a high resource usage rate.
Interference between users is a concern in LTE systems, not only at the edge of cell, but also in the cell. To reduce this interference, beamforming smart antenna techniques are used, e.g., within time division synchronous code division multiple access (TD-SCDMA) systems and may effectively reduce interference between users and provide interference coordination capability. Beamforming is a signal processing technique used to directionally shape antenna transmission patterns to avoid interference. Recently, for example, eight antenna beamforming smart antenna technology has been introduced into the TDD-LTE system. The eight antenna beamforming smart antenna technology makes it is easier to upgrade TD-SCDMA systems to TDD-LTE resulting in reduced network construction.
The 3rd Generation Partnership Project (3GPP) provides technical specifications for a 3rd generation mobile system based on evolved Global System for Mobile Communications (GSM) core networks. In 3GPP, release 8, there are seven transmission modes in an LTE system, every transmission mode related to a different radio condition and scenario. A specific transmission mode is defined to support beamforming in TDD-LTE. The other transmission modes are single-input and single-output system (SISO), transmission diversity, MIMO and multi-user MIMO (MU-MIMO).
In 3GPP release 8, a one-layer beamforming smart antenna is used, eight antennas sending and two antennas receiving (8×2). In 3GPP release 9, a dual-layer beamforming smart antenna is used in TDD_LTE systems, eight antennas sending and eight antennas receiving (8×8). However, not all the transmission modes need to use all eight antennas. For example, in a 2×2 multi-user (MIMO) mode only two antennas are used, while in a single-input and single-output system (SISO) mode only 1 antenna is used. Since in these modes specific antennas will not send any data at various times, the resource can not be used effectively, e.g., maximally.
Smart antenna beamforming is therefore not suitable for all transmission modes and conditions in LTE. For example, when radio conditions are sufficiently good and the number of users small, the system may not necessarily need to enable beamforming, so eight antennas are not necessary for sending data.
Typically, nodes using eight antennas to transmit SISO data and 2×2 MIMO data use two groups of 4 antennas. Such nodes may not fully use the space diversity gain, and as such may use more energy resources. What is needed is an effective way to schedule n antennas when less than n are needed (e.g., eight antennas when less than a maximum number of resources is needed, e.g., 2×2 MIMO or SISO). Effective scheduling of the antennas can improve downlink (DL) and uplink (UL) transmit diversity, create power savings, DL power control, interference reduction and signaling performance.